In recent years, bandwidths of communication networks are becoming wider, so that a greater amount of traffic has become circulated via the Internet. Especially, content distribution systems of the progressive download type are in widespread use, in which data processing is performed in parallel while downloading content having a large file size. In such a content distribution form, since data is transmitted via the network, data does not arrive at the terminal with a constant rate due to delay variation, packet loss, retransmission and the like, so that fluctuation occurs in transmission timing when data arrives. Therefore, there are many cases in which a mechanism is used in order to hide the effect of the network for the upper layer application by preparing a playback buffer for absorbing the fluctuation in the terminal side.
The content distribution form is characterized in that a certain amount of buffering (initial buffering amount) is necessary until start of processing, and the effect of the network can be relieved by increasing the buffer amount. On the other hand, the time until start of data processing becomes long, so that the response becomes worse. Therefore, it is necessary to set an initial buffer amount of a proper size in consideration of various situations. For example, there is a method in which the initial buffer amount is changed based on transfer delay time of packets (refer to a non-patent document 1, for example).
On the other hand, in a case where throughput of the network temporarily becomes lower than the processing speed of the content data, the playback buffer becomes lower than a predetermined amount (playback halt threshold), so that there is a case where playback is halted. The halt of playback causes a big effect on user sensory quality of the content. But, since the conventional technique cannot solve the problem, a technique is required for estimating and ascertaining playback halt states of content.
Also, as distribution methods of content, there are many schemes for, even for a single piece of content, dividing content into a plurality of units for distribution. In these distribution schemes, each divided unit is called a chunk, and content data is stored in the playback buffer in units of chunks. That is, the playback buffer is managed by the number of chunks or the playback time length for the stored chunks.